1. Field of the Invention
This invention relates generally to an automatic line monitor ("ALM") and more particularly to a monitor to control a switch that automatically accommodates line voltages in either the 110 volt range or the 220 volt range.
2. Description of the Prior Art
Some electronic power supplies operate at various AC input voltages, such as 110 volts and 220 volts. Some of these power supplies provide a jumper from a full-wave bridge rectifier that enables or disables a voltage doubler in the form of a pair of series-connected capacitors. With the jumper installed, rectified input voltage is applied in turn to each of the capacitors, each during its respective half-cycle That is, during one half-cycle, one capacitor receives an input charge and, during the next half-cycle, the other capacitor receives an input charge. In this way, with output voltage taken across both capacitors, the power supply provides a doubled voltage. With the jumper removed, the full-wave bridge rectifier impresses the rectified input voltage across the series combination of the two capacitors, rather than to each in turn, and the power supply operates in a non-voltage-doubling mode.
Other power supplies provide a switch in place of the jumper. A portion of the power supply senses voltage level, whether in the 110 volt range or in the 220 volt range. If the power supply senses the 110 volt range, the circuit shuts the switch providing the voltage-doubling mode as previously described. If the power supply senses the 220 volt range, the circuit opens the switch (or leaves the switch open, depending on "fail safe" operation) and the power supply operates in a non-voltage-doubling mode. In this way, the power supply provides the same output voltage regardless of the input voltage range.
Some power supplies provide a solenoid operated switch to effect the mode of operation. However, solenoids can be rather large and expensive, they suffer from certain reliability problems, and they can dissipate a relatively large amount of power.
Other known power supply circuits employ a triode or triac to accomplish the switching function. However, known control circuits for these switching arrangements generally provide an inadequate tolerance band between voltage ranges and provide some uncertainty in the point at which the switch is (de)activated due to manufacturing variations in the switch and other circuit elements. Also, known control circuits for these switching arrangements tend to be large and dissipate too much power.